FIG. 11 is a circuit block diagram of conventional angular velocity sensor 5001 described in Japanese Patent Laid-Open Publication No. 2002-174521. FIG. 12 is a block diagram of driver circuit 6 and failure detector circuit 7 of angular velocity sensor 5001.
Vibrator 1 made of an H-shaped piezoelectric crystal includes a pair of driving arms 2 and a pair of sensing arms 3 provided at the opposite side of the pair of driving arms 2. Sensing arm 3 is provided with a sensor electrode.
One of driving arms 2 is provided with driving electrode 4, and the other one of driving arms 2 is provided with drive detecting electrode 5. Driver circuit 6 is electrically connected to driving electrode 4 and drive detecting electrode 5 of vibrator 1, and controls them to vibrate vibrator 1 with predetermined amplitude. Failure detector circuit 7 includes window comparator 8 and BIT logic circuit 9 for monitoring an output signal of window comparator 8. Detector circuit 10 amplifies an electric charge output from sensing arm 3 of vibrator 1, converts the charge into a voltage, and outputs the voltage as an output signal to the outside from input-output terminal 11.
An operation of conventional angular velocity sensor 5001 will be described below.
When an alternating current (AC) voltage is applied to driving electrode 4 of vibrator 1, vibrator 1 resonates and generates an electric charge corresponding to vibrating amplitude of vibrator 1 in drive detecting electrode 5 of vibrator 1. This electric charge is amplified and adjusted by driver circuit 6, and is input to driving electrode 4 to vibrate vibrator 1 with the predetermined amplitude. When an angular velocity ω is applied to vibrator 1 while vibrating, an electric charge is generated in the sensor electrode provided on the pair of sensing arms 3. The electric charge generated in this sensor electrode is converted into an output voltage by detector circuit 10, supplied through input-output terminal 11, and input as an angular velocity signal to an external device, such as a computer, which in turn determines the angular velocity.
A circuit pattern around the sensor electrode may break during an extended period of use In this case, conventional angular velocity sensor 5001 outputs a signal which does not correspond to the angular velocity.
FIGS. 13 and 14 are a side view and sectional views of another conventional angular velocity sensor 5002 disclosed in Japanese Patent Laid-Open Publication No. 10-73437. FIG. 15 is a circuit block diagram of angular velocity sensor 5002.
Vibrator 101 made of a piezoelectric mono-crystal includes vibration body 102, vibration body 103 in juxtaposition with vibration body 102, and connecting arm 104 connecting between vibration bodies 102 and 103. Vibration body 102 is provided with four driving electrodes 105. Vibration body 103 is also provided with two detecting electrodes 106. Drive detector circuit 107 includes power supply 108, offset adjusting circuit 109, driver circuit 110, synchronous detector circuit 111, and differential amplifier circuit 112.
An operation of conventional angular velocity sensor 5002 will be described below.
When an alternating current (AC) voltage is applied from driver circuit 110 to driving electrodes 105 of vibration body 102, vibrator 101 vibrates due to resonance, and the vibration is transmitted to second vibration body 103 via connecting arm 104. When an angular velocity is applied to vibrator 101 while vibrating, detecting electrodes 106 provided on vibration body 103 generate an output signal corresponding to the angular velocity. This output signal is supplied to synchronous detector circuit 111 through a phase adjusting circuit. Synchronous detector circuit 111 performs synchronous detection on this output signal by using the driving signal output from driver circuit 110 as a reference signal, and supplies it to differential amplifier circuit 112 via a smoothing circuit. Offset adjusting circuit 109 receives a voltage from power supply 108, and outputs an offset voltage. Differential amplifier circuit 112 amplifies a difference between the offset voltage and the voltage output from the smoothing circuit, and produces two outputs 191 and 192. A difference in the potential between outputs 191 and 192 is used to detect the angular velocity.
FIG. 16 illustrates waveforms of the driving signal applied to driving electrodes 105 and a detected signal output from detecting electrodes 106. Detecting electrodes 106 generate an undesired signal even when no angular velocity is applied to vibrator 101 if there is a mechanically induced leakage due to unbalanced mass of vibrator 101 or an electro-mechanically coupled leakage attributed to a positional deviation of any of driving electrodes 105 and detecting electrodes 106. More specifically, the detected signal contains electrostatic leakage L101 due to electrostatic capacitances among driving electrodes 105 and detecting electrodes 106, and aggregate leakage L102 resulting from combination of the mechanically induced leakage and the electro-mechanically coupled leakage discussed above.
FIGS. 17A and 17B illustrate a cross-sectional view and a side view respectively of vibrator 101. In conventional angular velocity sensor 5002, a bottom portion of vibration body 103 is trimmed in order to reduce the mechanically induced leakage and the electro-mechanically coupled leakage of vibrator 101. This changes the mass balance of vibrator 101 to eliminate the undesired signal generated by angular velocity sensor 5002.
When vibrator 101 has a small size according to a small size of angular velocity sensor 5002, vibrator 101 tends to damage in the process of trimming the bottom portion of vibration body 103, and this makes it not feasible to eliminate the undesired signal generated from angular velocity sensor 5002.